This invention relates to the general field of power generators, and more particularly to an improved method and system for connecting multiple power generators such as turbogenerators or fuel cells.
A power generator, such as a turbogenerator with a shaft mounted permanent magnet motor/generator, can be utilized to provide electrical power for a wide range of utility, commercial and industrial applications. While an individual permanent magnet turbogenerator may only generate 20 to 100 kilowatts, powerplants of up to 10 megawatts or greater are possible by linking numerous permanent magnet turbogenerators together. Peak load shaving power, standby power, and remote location (standalone) power are just some of the potential applications for which these lightweight, low noise, low cost, environmentally friendly, and thermally efficient units can be useful.
The conventional power control system for a turbogenerator utilizes an electronic power converter to produce constant frequency, three phase electrical power that closely approximates the electrical power produced by utility grids. Key aspects of such a power generation system are availability and reliability.
In standalone operation, the turbogenerator is not connected to a public utility grid network. Thus, the standalone turbogenerator must be sized to meet the largest conceivable load power requirement. To oversize the turbogenerator, however, typically increases the cost and reduces the efficiency of the installation.
Clearly, there is a benefit to have a range of turbogenerators with a small power capacity increment between each turbogenerator. While a range of power capacities will provide flexibility in power capacity, much of the benefits of mass production of the turbogenerator are lost. By producing a small range of turbogenerators with a wide increment of power capacity, and the capability for parallel connecting two or more generators, a wide spectrum of load power requirements can be accommodated with a small range of standalone turbogenerators that are mass produced.
In addition to economic advantages, the use of several parallel turbogenerators can offer a number of operational advantages. Some redundancy to turbogenerator failure is obtained since the failure of one or more turbogenerators will still leave one or more turbogenerators able to provide for at least some fraction of the total load requirement. In contrast, should a single large turbogenerator fail, no power can be supplied to the load until the failure is corrected.
Further, since turbogenerator combustion engines typically have a limited power range where they operate close to maximum efficiency and with minimum pollutant emissions, a single large turbogenerator can have low efficiency and high emissions when it is operated at a power load requirement below its most desirable operating region. With a parallel connection of smaller turbogenerators, some of the turbogenerators can be deliberately turned off to keep the remaining turbogenerators running in the operating region that provides maximum efficiency and minimum emissions.
Parallel operation of generators has been utilized previously in electricity supply networks. In most cases, the generators are based on synchronous electric machines which are driven by an engine running at a speed synchronous with the other generators in the network. Load sharing is often managed by controlling the excitation magnetic field used in each generator and is facilitated by the relatively high output impedance of a synchronous electric machine (typically 5% to 20% of the ratio of rated voltage to rated current). These devices have the capability to supply currents considerably in excess of the steady state rating for short periods of time which allows each synchronous generator time to adjust to changes in the load balance without shutting down due to excess current.
While synchronization is not a real problem with synchronous electric machines, it is more difficult where the generators operate at variable speeds and an electronic power converter is utilized to supply a regulated output voltage and frequency. Power converter characteristics, which typically have lower output impedance and can deliver less current in excess of the steady state rating, make them inherently more difficult to connect in parallel than synchronous machines. For example, the electronic power converter used in a turbogenerator may have output impedance of less than 4% and is not capable of supplying current much in excess of the steady state rating.
In order to provide for load sharing of parallel connected turbogenerators in both steady state and transient load conditions, it is extremely important to operate parallel connected power converters at the correct voltage, frequency, and phase angle. The magnitude and phase angle of the output currents in all of the parallel connected power converters must be similar under both steady state and transient load conditions and all of the power converters must operate at the same output frequency.
Prior art solutions to this problem generally fall into two categories; those systems where some form of signal level connection are used to exchange load sharing information between parallel connected power converters, and those systems which do not have this type of signal level connections between parallel connected power converters.
When signal level communications are used, they are typically employed to provide load current or power sharing information, transmitted in analog form as a voltage or current level. Often this information is the average output current or power level, where this average is taken across all of the parallel connected power converters. Signal level connections have also been used to provide frequency and phase synchronization information between the power converters or from some master control device.
When no signal level communications are used, each power converter is controlled in a manner similar to previous solutions for the parallel connection of synchronous generators. Each unit contains a control system that sets the output frequency and internal voltage magnitude in accordance with the measured current magnitude and phase angle with respect to the measured output voltage angle. These schemes do not offer as precise load sharing as those where additional load share information is passed using signal connections between power converters, but have the advantage of dispensing with the cost and reliability concerns associated with signal level connections.
There are also existing solutions that help increase the output impedance characteristics of parallel connected power converters to assist with load sharing during load transients. These schemes either involve the insertion of actual impedance in series with the output of each power converter or involve the use of feedback on the output current to modify the output voltage in accordance with a desired impedance characteristic. The most commonly applied technique is the use of negative current feedback to provide for a resistive output characteristic. In this case, the output voltage of the power converter droops as the input current increases. This voltage droop method provides for load current sharing in the steady state and under transient conditions. Among its disadvantages, however, is that the output voltage regulation/control is poor compared with other load sharing methods and the output voltage waveforms become distorted when supplying non-linear loads (which require a non-sinusoidal current).
The invention is directed to connecting a plurality of power generators, each having an electrical power converter output and adjustable output voltage magnitude and frequency, with at least one digital communications bus to provide synchronization and load sharing. One of the plurality of power generators is designated a master unit and the remainder are designated as slave units. The digital bus(es) passes voltage magnitude correction information, voltage harmonic correction information, and synchronization information from the master unit to the slave units.